The present invention relates to a vehicle hydraulic brake system comprising a master cylinder actuatable by a hydraulic power booster and containing a working chamber, to which wheel brakes are connected by way of valve means controllable by a slip control electronics, in which system on control action pressure fluid can be taken from the wheel brakes through the valve means and can be replenished out of the pressure chamber of the hydraulic power booster by way of the working chamber, wherein during communication of the working chamber with the pressure chamber, a resetting sleeve is pressurized in the brake's release direction which is penetrated by a master cylinder piston and which serves to limit the stroke of the brake pedal.
A device incorporating the preceding features is known from German printed and published patent application No. 3040562. This known brake system comprises a hydraulic power booster which, in turn, disposes of a pedal-actuatable brake valve. A pressure proportional to the respective actuating force can be metered from a pressure fluid reservoir into a pressure chamber of the hydraulic power booster, what, in excess of a minimum pressure of response in the pressure chamber of the hydraulic power booster, will finally result in displacement of the booster piston in the actuating direction. An axial displacement of the booster piston is first transmitted onto a first master cylinder piston of a tandem master cylinder so that a first working chamber of the tandem master cylinder will be pressurized. Said first working chamber of the tandem master cylinder is confined by a second master cylinder piston which will be shifted hydraulically in the actuating direction so that likewise a second working chamber of the tandem master cylinder will be pressurized. Wheel brakes of an automotive vehicle are connected to the working chambers of the tandem master cylinder.
Inserted in the connections between the working chambers of the tandem master cylinder and the wheel brakes are valve means which are controllable by a slip control electronics and which serve to interrupt the hydraulic connections between the working chambers and the wheel brakes. The wheel brakes of the automotive vehicle are in communication with an unpressurized supply reservoir via normally closed valve means which can be opened by the slip control electronics, so that pressure fluid is removable from the wheel brakes what allows to reduce the effective braking pressure and to cause re-acceleration of the vehicle wheels in the event of a control action.
The pistons of the tandem master cylinder are sleeve-sealed, and in each case one annular surface of the master cylinder pistons remote from the working chamber is during control action adapted to be acted upon by the dynamic pressure of the pressure chamber of the hydraulic power booster. Pressure fluid which is removed from the wheel brakes during brake slip control will be replenished this way out of the pressure chamber of the hydraulic power booster via the working chambers of the tandem master cylinder.
On exposure of the master cylinder pistons to the dynamic pressure, a resetting sleeve is simultaneously acted upon in the brake's release direction, in which sleeve one of the master cylinder pistons is sealedly guided. Said resetting sleeve will displace on dynamic pressurization so far in the brake's release direction that an annular collar of the master cylinder piston moves into abutment on the pedal-side annular surface of the resetting sleeve so that the axial displacement of the master cylinder pistons in the actuating direction is limited.
In the brake system described, the relatively complicated design has to be regarded as less favorable on the whole. Furthermore, a dynamic pressurization of the resetting sleeve may ensue abrupt resetting movements of the brake pedal, whereby driving comfort is lessened.
Therefore, it is an object of the present invention to improve upon a vehicle hydraulic brake system of the species initially referred to with constructively simple means such as to delay the resetting movement of the master cylinder piston.